FM radio is in wide use in the field of radio broadcast. The term FM includes, for example, any of the Frequency Modulation methodologies used or developed for signal broadcasting in a frequency band assigned by the U.S. Federal Communications Commission (FCC), nominally in the transmission range 88 MHz to 108 MHz, which is near the middle of the Very-High-Frequency (VHF) television broadcast band. These Frequency Modulation technologies include both analog FM and digital FM.
The FCC has adopted a standard for analog-digital FM transmission called the iBiquity IBOC (In-Band-On-Channel) for hybrid analog-digital transmission systems. According to the IBOC standard, FM stations in the United States must be able to simultaneously broadcast analog and digital signals within their current allocated frequency range. One approach for achieving the above simulcast is to use two separate transmission systems (for example, analog-digital) to feed two separate antennas (for example, analog-digital). Since the elevation of the antenna on the tower directly affects the antenna's coverage, it would be desirable to co-locate the radiated analog and digital signals at the same height above the ground to maintain the same coverage.
Also, since the azimuthal pattern of an FM antenna is very dependent on the cross section of the tower structure, it would be desirable to mount both the analog and digital antennas in the same orientation with respect to the tower. When adding digital coverage, concerns are that many towers are already full having no additional aperture space available. Therefore, many FM broadcasters have responded by vertically interleaving the second digital antenna within the aperture of their existing analog antenna.
A technical challenge in the broadcast community is the fact that the analog and digital signals occupy the same band width, but require isolation between the two systems. Current requirements for isolation between the IBOC digital and analog signals is on the order of 35 dB. If the IBOC and analog antennas are in the same aperture, in order to reduce the filtering requirements the highest level of antenna isolation is preferable. Herethereto, conventional approaches to this dilemma have resulted in antenna systems using sophisticated signal conditioning schemes, pattern manipulation orthogonization, and isolation equipment. All of these approaches rely on the use of similar radiators, in conformity to legacy systems and FCC regulations.
However, the FCC has issued a Public Notice DA04-712, dated Mar. 17, 2004, authorizing the use of separate antennas for digital FM transmissions that are within three geographical seconds of the current analog site. The FCC's public notice on the approval of separate antennas to initiate digital FM digital transmissions explicitly enables the broadcaster to use a separate digital antenna on a separate tower—thereby ameliorating the current dilemma of adding a digital antenna to an existing overcrowded analog tower.
However, the use of a separate tower dedicated to digital systems unavoidably incurs significant tower, cabling, and material costs, as well as the resources necessary to maintain twice as many towers as in the analog operation.
Thus, in view of the FCC's public notice, alternate schemes and techniques for providing FM transmission are desirable. Specifically, schemes or techniques that reduce the isolation concerns without requiring a separate tower.